PID? 1

[banwart]

Hello,
I am working on a control project that will need temperature control and a recipe. The control on the temperature does not need to be really tight as we are going to boil mixture of mostly water for 3minutes at a time. We have a steam boiler and an air actuated pressure valve(4-20mA) to a jacketed 200 gal mixing tank. Also I have an RTD into a Micrologix1500 w/RTD I/O card for my temperature readings. I'm using RSLogix500 and have just recently been reading about the PID instruction. My main concerns are finding the correct PID settings to start testing from, and then using the instruction in my program correctly as I would like to tie the PID setting to a certain Recipe selection.
Anyone who could give pointers or tips on using PID or RCP I would appreciate the ideas. There seems to be a lot of theory about PID to read I'd like to see some examples if anybody has some to share. Also I'm using a 10" EZTouch color panel for the HMI. Anybody else out there using this type? Thank you.

 

[skills]

Alright, begin by setting the integral and derivative portions to 0 or turned off. Then using only the P term, set desired S.P. and increase P until the PV just approaches the S.P. without entering a steady state oscillation. Measure the 'droop' or difference S.P. - PV and reduce the P variable to increase the droop by 1.85. Then increase the I variable to arrive at S.P. in the desired time without entering steady state oscillations. The D should only be used if the I required above causes loss of control when rapidly changing S.P. I believe this will get you set up quickly.

 

[xnuke]

Learn as much as you can about PID tuning methods. There is a lot available on the web concerning PID tuning. Read as much of it as you can stand. Personally, I prefer to use the Ziegler-Nichols open loop tuning method when commissioning a control loop as opposed to closed loop methods that require approaching oscillation in my system. You can get both system identification information as well as tuning parameters from this method. It also avoids most chances of catastrophic failure since only small changes are made and the system does not approach instability.

By the way, do you have any controls engineering experience? If not, you may want to consult an expert, or at least find someone at your company with experience to help you. Control engineering requires a wide range of knowledge drawn from electrical, mechanical, and chemical engineering, with networking and data management thrown in for good measure. It's not just about connecting equipment together with wiring and piping to make a system - the system has to work according to specifications, as well as allow interfacing for control and data viewing.

Good luck.

xnuke

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[banwart]

Yes xnuke, I have some prior experience in Electrical and Control Engineering. I just started with this particular company, I've spent the last few years in the OSI model (I won't deny that I'm much more up to date on Computer and Network Engineering.) So I am just looking at getting peoples thoughts and experiences with PID instruction. Thanks for the input.

 

[plcsavvy]

The type of machinery that I design controls for does not require the use of PID control. However, I do occasionally use the EZ-TOUCH operator interfaces. There are very inexpensive as Compared to TCP and Allen bradley, Xycom and others. I have interfaced the EZ-TOUCH with Allen-Bradley, and Automation Direct Koyo 305 PLCs. This screen is usally reserved for simple machines that don't require any fancy batch processing or data storage and retrieval.

Best regards, PLCSAVVY

 

[laguna]

I agree with xnuke, I also prefer to use the Ziegler-Nichols tunning. For the beginning, you could look for some articles of John A. Shaw.
Luck.

 

[nbucska]

I strongly suggest you to write a computer model -- even if
only in BASIC -- to gain a more perfect understanding.

<nbucska@pcperipherals.com>

 

[nbucska]

I strongly suggest you to write a computer model -- even if
only in BASIC -- to gain a more perfect understanding.

PLC is not the easiest to program more complex algorithms
in -- wouldn't be bang-bang control good enough ?


<nbucska@pcperipherals.com>